Allow users to create their own C++ views for Qt Quick

For QML and Qt Quick to be able to create advanced applications developers need to be able to create custom views for their models, rather than hoping that there's an existing view that can be adapted for their purposes.

For example, I want to create a Repeater/ListView-style element that inserts a separator between each of the delegates (except before the first and after the last). This can't be easily done with any of the existing components so I'm having to depend on private classes to implement my own.

This has been a recurring issue throughout the years, with people wanting to be able to build:

• Masonry views (QTBUG-57549, QTBUG-85397)
• KDE wanting to build a desktop overview (QTBUG-115804)
• Other things like a photo grid with different image sizes

There are more use cases, but the point is that it's better if we're provided with the low-level primitives to build whatever we want, rather than hoping a new component is provided to cater to our needs.

Having things like QQmlChangeSet, QQuickFlickable, QQuickTransition and QQmlDelegateModel public would allow people to start building all sorts of cool views that can efficiently update when models update.